1. Field of the Invention
This invention relates to field emission displays (FEDs) and more specifically to a light modulator technology that employs a field emitter array (FEA) to address a charge controlled mirror (CCM)
2. Description of the Related Art
Image displays are used to convert electrical signals into viewable images. The most common technology used in both projection and direct-view displays is the cathode ray tube (CRT), in which a scanning electron gun shoots a single beam of electrons across a vacuum to scan a phosphor-coated anode. The electrons penetrate the individual phosphors causing them to emit light and taken together produce a direct view image. By necessity, the gun must sit far from the anode to raster scan the phosphor screen, a distance similar to the width of the display area. As a result, high-resolution large area direct-view displays are correspondingly very large and very heavy.
During the past 40 years numerous attempts have been made to construct a "Flat-CRT", which can overcome the length and weight limitations of the conventional CRT without sacrificing performance. With few exceptions, these efforts have failed commercialization due to serious complexities in the electron source and mechanical structure, but a new alternative called the Field Emission Display (FED) has recently appeared that has shown promise in overcoming these barriers. The FED utilizes a matrix addressed cold cathode array, spacers to support the atmospheric pressure, and cathodoluminescent phosphors for efficient conversion of the electron beam into visible light. The non-linearity of the current/voltage relationship permits matrix addressing of high information content displays while providing high contrast ratio.
The FED combines the best properties of CRTs (full color, full greyscale, brightness, video rate speeds, wide viewing angle and wide temperature range) with the best attributes of Flat Panel technology (thin and light weight, linearity and color convergence). However, the current production FEDs have limited display sizes, 10 inch diagonal or less, due to the fabrication and vacuum packaging problems. Since the primary motivation for Flat-CRTs was to overcome the size and weight limitations of the conventional CRT for large display sizes, this is a serious problem to successful commercialization of the FED technology.
To appreciate FEDs, one must understand the physics of field emission. The potential barrier at the surface of a metallic conductor binds electrons to the bulk of the material. This potential barrier is called the work function, and is defined as the potential difference between the Fermi level and the height of the barrier. For an electron to leave the material, the electron must gain an energy that exceeds the work function. This can be accomplished in a number of ways, including thermal excitation (thermionic emission), electron and ionic bombardment (secondary emission), and the absorption of photons (photoelectric effect). Fowler-Nordheim emission or field emission differs from these other forms of emission in that the emitted electrons do not gain an energy that exceeds the material work function.
Field emission occurs when an externally applied electric field at the material surface thins the potential barrier to the point where electron tunneling occurs, and thus differs greatly from thermionic emission. Since there is no heat involved, field emitters are a "cold cathode" electron source. One needs to apply an electric field on the order of 30-70 MV/cm at the surface of a metallic conductor to produce significant tunneling current. For example, if an electrode is placed 1 um from the surface of a conductor it would take 1000 V between the electrode and cathode to induce significant current flow.
Obviously, a flat-panel display (FPD) that is addressed at 1000 V is of little use. Therefore, "field enhancement" is used to lower the necessary addressing voltages.
A field emitter is a sharp point, or whisker, with a connecting cathode electrode, a dielectric layer, and an isolated extraction gate in close proximity. If a positive potential is applied between the gate and cathode, a uniform electric field is produced in the dielectric. But the presence of the sharp tip emitter produces a compression of the equipotential lines at the tip, and thus a high electric field. Field enhancement is a geometric property and is strongly dependent on the sharpness of the tip. Note that the dielectric must hold off the un-enhanced field, so field enhancement is essential for operation of field emitters. With field enhancement, a reasonable voltage applied to the extraction gate results in electron emission at the point.
As shown in FIG. 1, a vacuum packaged FED 10 includes a matrix-addressed cold cathode array 12, spacers 14 that support atmospheric pressure and a cathodoluminescent anode 16. Cathode array 12 is composed of row and column conductors separated by an insulating layer (not shown) with interspersed field emitter tips 17. These layers are deposited on an insulating substrate 18, such as glass. The locations where the rows and column cross define a pixel. The row conductors serve as the extraction gate and the column conductors connect to the cathodes.
Anode 16 is the phosphor screen and is composed of phosphor powders 20, which are typically deposited within a black matrix on a glass substrate 22. The entire anode 16 is covered with a thin aluminum layer, which bleeds off the electrons that bombard the screen and returns them to the power supply. The cathode and screen, along with spacer materials, are aligned, sealed, and evacuated to complete the vacuum package.
Electron emission from each pixel is controlled by a forward bias between the gate and cathode. Once released from the confines of the bulk material, the emitted electrons are accelerated toward the phosphor screen. A focusing grid (not shown), which is biased at a positive potential with respect to the cathode, is often used to focus the electrons as they are accelerated toward the screen. The voltage applied to the screen must be higher than the cathode voltage or the emitted electrons. The screen voltage must also be high enough so that most of the electrons'energy remains once they penetrate the aluminum layer covering the phosphor particles.
As shown in FIG. 1 and in more detail in FIG. 2, drive electronics 24 are needed to control operation of the vacuum package and specifically cathode array 12. The drive electronics subsystems include a power module 26, a video controller 28, panel controller 30, and row and column drivers 32 and 34, respectively. The component subsystems will differ depending on whether the input is analog or digital.
For an analog composite video signal containing red, green, and blue (RGB) information and timing signals, video controller 28 samples the analog video signal, digitizes it, and separates it into RGB components. Horizontal and vertical timing information is also extracted from the composite input. Video controller 28 then presents the digitized video information to panel controller 30 in the form required by a standard digital video interface specification. This standard specifies digital RGB data up to 18 bits in parallel, horizontal and vertical sync, a pixel clock, and a data valid signal. Other processing that may be required in the video controller are gamma correction and adjustment of color saturation, brightness, and contrast.
In order to keep the FED compatible with other FPD technologies that accept digital input, panel controller 30 must accept the standard digital-interface signals and extract the signals necessary to drive the FED row and column drivers 32 and 34. In most cases, the signals appearing at the digital interface are used directly by the row and column drivers, and the functionality of the panel controllers is minimal. However, depending on the drive approach used and on the design of the drivers, some functionality may be required on the panel.
Line-by-line addressing is used to display an image on the FED. Typically, the row connections are the FED gates, and the columns are the FED cathodes. The rows are scanned sequentially from top to bottom. As each row is selected, the columns are used to modulate the current in the pixels of the selected row. This results in dwell times much longer than those produced by the flying spot of a conventional CRT. The longer dwell time permits lower pixel current for a given brightness, thus eliminating the problems of beam divergence and phosphor saturation that occur in high-brightness CRT's.
The voltage applied across the pixel is the difference between the row-select voltage and the column voltage. For a typical FED, a gate-cathode voltage of approximately 80 V is required to achieve full "white" brightness. The pixel OFF current for black level is 50 V or less. The modulation voltage used to control the intensity of each pixel is the difference between the white and black levels, or about 30 V. From a functional standpoint, the row driver is a very simple circuit that provides only a row-select signal as the display is scanned from one line to the next. The column driver presents gray-scale image information to the pixel and differs from the row driver both in functional complexity and bandwidth performance.
There is more than one way to modulate the pixel intensity with the column driver, and there are tradeoffs with each approach, including power consumption, susceptibility to cathode defects, ability to drive the required load, and display uniformity. The leading approaches are amplitude modulation (AM), pulse-width modulation (PWM), and a mixed AM/PWM approach. Each of these approaches can be used with column drivers configured as either voltage or current sources.
Although Flat-CRTs have been demonstrated and produced in limited quantities using FED technology, the FED industry faces serious problems in the fabrication and vacuum assembly of the field emitter arrays due to the inherent limitations of emissive displays. To get a very bright display the phosphors must be driven at high power levels, which shortens the phosphor lifetime dramatically. It is well known in the projector arena that phosphor displays reach their one-half brightness level after the first year of use. In addition, the alignment of the RGB phosphors for a color display can be tricky. Furthermore, the voltages required to penetrate the aluminum coating and operate the phosphors at these levels shorten the expected lifetime of the field emitters. Due to this rapid aging FEDs are not suitable for projection displays.
As a result, FEDs are currently limited to direct-view displays such as television and computer displays, in which 27 and 17 inch and larger displays are quite common. Unfortunately the thin and thick film processes used to fabricate the cathode and anode structures, respectively, are incompatible. It is very difficult to marry the clean thin-film process with the dirty thick-film process to produce a clean device on which a vacuum can be pulled and maintained over the lifetime of the display. The large display sizes and high resolutions required to meet consumer demand exacerbate this problem by increasing the total surface area of the phosphors, hence the number of hiding places for contaminants that can out gas over time.
The spacers in a FED must be mechanically strong and stable, be compatible with a surrounding vacuum and have a high breakdown voltage. In addition, their electrical resistance must be high enough to minimize leakage current between anode and cathode. Yet the resistance has also to be low enough for charge buildup to dissipate. Currently, the spacers are fabricated separately and then positioned on the anode using a robotic pick and place procedure, which is time consuming and very expensive. The described packaging and performance limitations have impaired the industry's ability to produce an FED having a large display area that is very bright and maintains that brightness over its lifetime.
Leard et al., U.S. Pat. No. 5,196,767 discloses an optical signal processor that uses a matrix-addressable field emitter array to supply controlled electron emission to a two-dimensional signal processor element such as a deformable reflective membrane as described in U.S. Pat. No. 5,287,215 or a liquid crystal array. The optical signal processor is particularly suited for applications in adaptive optics, optical computing, target recognition, tracking and signal processing and optical communications.